A conventional hearing aid typically comprises a housing that defines a generally closed cavity therein in which are arranged a power source, an input transducer, for example, a microphone, and associated amplifier for transforming external sounds into electrical signals, a signal processor for processing the transformed signals and producing signals optimized for particular hearing losses, and an output transducer, called a receiver, for transforming the processor signals into hearing-loss compensated sounds that are emitted into the ear. A hearing aid typically also comprises respective sound tubes extending from the input port of the microphone or the output port of the receiver to the housing outside surface to establish acoustic pathways between the microphone and the outside surroundings and between the receiver and the ear canal, respectively.
Hearing aids may be constructed to be wearable in the ear (for example, in-the-ear (ITE); in-the-canal (ITC), and completely-in-the-canal (CIC) hearing aids). For this type of hearing aid, the ear canal is either partially or completely closed off from the surroundings outside the ear. So-called “occlusion effects” are a consequence of this occlusion of the ear canal. Specifically, there occurs a pressure build-up in the residual volume of the unblocked portion of the ear canal, defined by the hearing aid and the ear drum, from the sound emitted by the vibration of the tissue in the ear canal that is normally caused by the voice of the hearing aid user. The voice of the hearing aid user becomes amplified and hollow and dominates the sounds reaching the ear drum. This results in poor sound quality of the user's own voice as well as the other sounds reaching the ear drum.
There are several ways to diminish or reduce these occlusion effects. For example, a hearing aid may be configured to have at least one ventilation channel or passage (“vent”) that extends from the portion of the hearing aid housing facing the residual volume to the portion of the hearing aid housing facing outside the ear. The vent facilitates transmission of acoustic energy from one side of the hearing aid to the other so that the ear canal is not completely blocked. The vent thus reduces occlusion effects by, first, providing a passageway to permit the body-conducted portion of a user's own voice to dissipate and, second, equalizing the atmospheric pressure between the air in the outside surroundings and in the residual volume. One of the disadvantages of a vent, however, is that the vent also provides an acoustic bypass to the normal signal path via the hearing aid components (for example, the microphone, the signal processor, and the receiver) that may hamper the operation of the hearing aid, causing, for example, feedback instability and a reduction of directionality for directional hearing instruments (this is further described in an article by J. Mejia, H. Dillon, M. Fisher, entitled, “Active cancellation of occlusion: An electronic vent for hearing aids and hearing protectors”, J. Acoust. Soc. Am. 124 (1), July 2008, pp. 235-240, which is incorporated by reference herein).
More recently, hearing aids have been constructed with active occlusion reduction (AOR) circuitry. U.S. Patent Publication 2008/0063228 (“Mejia, et al.”), which is incorporated by reference herein, shows a hearing aid having AOR circuitry that reduces occlusion by electro-acoustic means. Hearing aids with AOR circuitry generally comprise a second input transducer (referred to as an “AOR microphone” or “internal microphone”) that is located inside the hearing aid housing facing the residual volume of the ear canal and that picks up all sounds, including occlusion sounds in the residual volume. The picked-up sounds are processed and combined with the processed external sounds picked up by the external microphone. The hearing aid having AOR circuitry treats the occlusion sounds in the residual volume as an error in a closed-loop feedback system. In particular, the hearing aid having AOR circuitry uses the occlusion sound signals to generate compensating sound signals (“anti-occlusion signals” or “occlusion-negating sounds”) that are projected by the receiver into the residual volume (which also projects the hearing-loss compensated sounds). The occlusion sounds in the residual volume get compensated as they combine with occlusion-negating sounds that the hearing aid generates. A hearing aid having AOR circuitry is typically still configured to have a conventional vent as well, with comparatively small dimensions, not to address occlusion reduction directly but to provide frequency response stability and balance barometric pressure differentials.
However, due to the limited bandwidth of hearing aid AOR transducers (specifically, the receiver and the AOR microphone) as well as processing delays, one adverse effect of a hearing aid having AOR circuitry is that the negative feedback of the closed-loop AOR system at 100-1000 Hz turns into positive feedback below 100 Hz, creating a gain boost between 10 and 100 Hz. A well-tuned and optimized hearing aid having AOR circuitry typically has a resonance peak of 5-10 dB between 10 and 100 Hz. As a result, sound in the frequency range of the resonance peak which is entering the hearing aid is amplified. This low frequency amplification is perceived as a very annoying artifact to the user.
Hearing aids with a vent or AOR circuitry or both also may be adversely affected by walk-induced head vibrations (WIHV). This is described in detail in Technical Bulletin TB5 by Knowles Electronics, Inc. entitled, “Walk Induced Head Vibrations and Hearing Aid Design”, pp 1-4 (not dated). The Technical Bulletin describes walk induced head vibrations (WIHV) and its consequences for the operation of a hearing aid, specifically pointing out as a problem a “ . . . resonance between 20 and 30 Hz due to the head mass resting on the neck stiffness . . . .” A hearing aid with a conventional vent may be affected by walk induced head vibrations. In particular, the external microphone may pick up the vibrational energy and convert it to signals that could overload the hearing aid circuitry and the receiver, thereby, creating distortions. A hearing aid with AOR circuitry is much more sensitive to WIHV because such vibrations create a sound pressure inside the residual volume of the occluded ear canal. The internal microphone can pick up the vibrational sound pressure and feed it to the AOR circuitry that, as noted above, has a resonance between 10 and 100 Hz. As a result, the AOR circuitry gets overloaded by WIHV signals and creates strong audible distortions.